This was not needed before, but now it seems to be necessary for
some reason. The code is just an adjusted copy of the appropriate
piece of the OLE2 protocol code, sending GDK_SELECTION_REQUEST.

The rest is just fixing the fallout, allowing LOCAL protocol to pass
the functions it wasn't supposed to pass before.

Closes #82

Now that subtitle's default value "Searching" for OPERATION_MODE_SEARCH
is duplicated as it should be, we cannot reassign other strings to it
anymore, as that resulted in the original dupe of "Searching" leaking.

Fix this by only assigning the dup'd "Searching" after trying to get
more specific values, not before. We therefore need to set it to NULL
during its declaration, and that means we needn't in the final else.

Having a FileChooserDialog in location-entry mode then pressing
<primary>f to move to search mode would crash with an invalid free().

In that case, FileChooserWidget.get_subtitle() returned a static string
straight from gettext. This crashed when the GBinding from :subtitle to
FileChooserDialog’s HeaderBar:subtitle shortly tried to free the string.

Fix by duplicating the string before returning it, like all other paths.

https://bugzilla.gnome.org/show_bug.cgi?id=791004